FIG. 1 shows a basic circuit diagram of such a PFC according to the prior art. The task of a PFC is to make available, from an input voltage Vin, a rectified, at least approximately load-independent output voltage Vout which serves for supplying a load, for example a DC-DC converter. The PFC comprises input terminals K10, K11, at which a mains AC voltage is present as input voltage Vin, the root-mean-square value of which may be between 90 V and 270 V in the case of a long-range power supply. The PFC comprises a bridge rectifier BG, and a further rectifier arrangement connected between the bridge rectifier BG and the output terminals K20, K21 and having a series circuit formed by a coil L and a diode D and a capacitor C connected between the output terminals K20, K21 and also having a semiconductor switch T, which is connected between a node that is common to the coil L and the diode D and the bridge rectifier BG and which serves to connect the coil in parallel with the output terminals of the bridge rectifier.
The semiconductor switch T is driven in clocked fashion by a drive IC which, via the semiconductor switch T, serves for controlling the power consumption of the PFC and thus for setting the output voltage Vout. For this purpose, a signal S1 dependent on the output voltage Vout is fed to the drive IC. During operation, the coil L stores energy when switch T is closed and outputs said energy via the diode D to the output capacitor C and a connected load when switch T is subsequently open. In this case, the power consumption is dependent on the duty ratio of the switch T, that is to say on the ratio of switch-on duration to drive period. In this case, the drive period is usually prescribed by an oscillator signal generated in the drive circuit.
The duty ratio of the switch T varies over the period of the usually sinusoidal input voltage, the frequency of which is significantly less than the switching frequency. This variation results from the fact that, in the case of a high instantaneous value of the input voltage, a shorter switch-on duration of the switch T suffices in order, per drive period for the maintenance of a required power consumption, to take up the same quantity of energy as in the case of a lower instantaneous value of the input voltage Vin. Overall, it holds true that as the root-mean-square value of the input voltage decreases, the duty ratio increases in order to achieve a given power consumption. The same holds true in the event of a rise in the power consumption of a load connected to the output terminals. In this case, too, the duty ratio of the switch increases in order to increase the power consumption overall for a given input voltage.
In the case of PFC with power consumptions above a legally prescribed limit value, there is the requirement for a mains current consumption which is proportional to the input voltage. For a control of this type, a signal dependent on the input current is also fed to the drive IC in accordance with FIG. 1. An example of a drive circuit for such a PFC is a module of the TDA 16888 type from Infineon Technologies AG, Munich.
During operation of a PFC, critical operating states may occur if, by way of example, the output voltage decreases to a value which lies outside the dimensioning limits of the switching converter. This is the case for example in the event of a mains failure or a so-called “brownout”. “Brownout” denotes a disturbance in the supply mains in which the mains voltage falls, over a plurality of mains periods, below a limit specified for the power supply, said limit being 90 V, for example in the case of a long-range power supply.
The control of the PFC attempts to compensate for this decrease in the mains voltage by long switch-on periods of the switch T, in order to maintain a required power consumption. However, this can lead to impermissibly high currents in the switch T for which it is not designed. In order to avoid such high currents, PFCs with current limiting are known in which the switch T is switched off when its load current exceeds a maximum value. This has the effect that the switch T carries current only for in each case a short time, but the diode D carries current in each case for a correspondingly longer time, so that an impermissible heating of the diode D may result from this.
In order to avoid this problem, it is known to detect the root-mean-square value for the instantaneous value of the input voltage and to switch off the PFC if said root-mean-square value or the instantaneous value falls below a lower limit value. Such a PFC which detects the instantaneous value of the input voltage is disclosed for example in the data sheet UCC3817 from Unitrode Inc. What is disadvantageous in this case is that detecting the input voltage requires a voltage divider which divides the input voltage down to values which can be processed by a logic circuit. The components required for the voltage divider increase the costs and the circuitry outlay of such a PFC. Furthermore, detecting the root-mean-square value of the input voltage has the disadvantage that the input voltage has to be averaged over half a period of its temporal profile, which requires an integrator circuit having a large time constant, which can only be realized by means of external components. However, such external components considerably increase the costs and realization outlay of such a circuit.
It is an aim of the present invention to provide a drive circuit for a switch in a switching converter, in particular in a PFC, which ensures a protection of the switching converter in the case of critical operating states, in particular in the case of a decrease in the output voltage, and which does not require detection of the input voltage. Furthermore, an aim of the invention is a method for driving a switch in a switching converter, in particular in a PFC.